Little by little, the world's easily found and easily produced petroleum energy reserves are becoming exhausted. Consequently, to continue to meet the world's growing energy needs, ways must be found to locate and produce much less accessible and less desirable petroleum sources. Wells are now routinely drilled to depths which, only a few decades ago, were unimagined. Ways are being found to utilize and economically produce reserves previously thought to be unproducible (e.g., extremely high temperature, high pressure, corrosive, sour, and so forth). Secondary and tertiary recovery methods are being developed to recover residual oil from older wells once thought to be depleted after primary recovery methods had been exhausted.
Some crude oils (or, more broadly, reservoir fluids) have a low viscosity and are relatively easy to pump from the underground reservoir. Others have a very high viscosity even at reservoir conditions.
Sucker rod pumps may be utilized to lift viscous crude oils, but in many fields, sucker rod pumps cannot be used. For example, sucker rod pumps are not feasible in highly deviated wells. In many fields, limited surface rights make sucker rod pumps unfeasible. Offshore production must be accomplished from platforms which are expensive and have limited space available for pumping units.
Electrical submersible pumps are often used when sucker rod pumps are not feasible, but electrical submersible pumps can only pump crude oils of a viscosity of about 200 cs or less. This represents crude oils having API gravities of greater than about 12.degree. API.
U.S. Pat. Nos. 4,832,127 and 4,749,034 disclose apparatus and processes to produce viscous crude oils from wellbores utilizing electrical submersible pumps. These inventions mix water with the crude oil at relatively high shear rates to force an emulsion to form at the inlet to the pump. The emulsion has an effective viscosity less than the viscosity of the crude oil. These inventions make it possible to produce oils otherwise not producible by electrical submersible pumps, but an excessive amount of water injection is required. For example, the process of U.S. Pat. No. '127 utilizes from 300 to 1,200 barrels a day of water to produce about 225 barrels a day of oil. This excessive amount of water results in larger pumps, motors, and surface separation equipment. Further, because an emulsion is created, surface separation equipment must be capable of breaking the emulsion.
Methods to establish core flow in pipelines are disclosed in, for example, U.S. Pat. Nos. 3,886,971, 3,977,469, 4,047,539, 4,745,937, and 4,753,261. These processes establish a core flow of a viscous fluid within a core of a less viscous fluid in order to reduce the pressure drop in the pipeline. An apparatus and process to consistently create core flow in an inlet to a submersible electric pump is not taught or suggested in these references. Further, these references do not teach or suggest that the significant problems encountered by electric submersible pumps in pumping viscous oils, i.e., motor cooling and low pump efficiencies, can be overcome by establishing core flow at the inlet of the electrical submersible pump. It is not uncommon, therefore, for example in California, to find wells with considerable quantities of valuable crude which have nevertheless not been producible because it was too expensive to produce the viscous crude.
It is therefore an object of the present invention to provide a method and an apparatus to lift viscous oils from wellbores while injecting water at a rate less than about 25 percent by weight of the total flow rate. It is a further object to provide a process and an apparatus which utilizes an electrical submersible pump to lift viscous oils from wellbores and results in electrical motor temperature rises of less than about 20.degree. F., and pump efficiencies of greater than about 50 percent pump efficiency and greater than about 80 percent of the pump water efficiency.